Studies have found that ships have a net cooling effect on the planet, despite belching out nearly a billion tons of carbon dioxide each year. That’s almost entirely because they also emit sulfur, which can scatter sunlight in the atmosphere and form or thicken clouds that reflect it away.

In effect, the shipping industry has been carrying out an unintentional experiment in climate engineering for more than a century. Global mean temperatures could be as much as 0.25 ˚C lower than they would otherwise have been, based on the mean “forcing effect” calculated by a 2009 study that pulled together other findings (see “The Growing Case for Geoengineering”). For a world struggling to keep temperatures from rising more than 2 ˚C, that’s a big helping hand.

And we’re about to take it away.

In 2016, the UN’s International Maritime Organization announced that by 2020, international shipping vessels will have to significantly cut sulfur pollution. Specifically, ship owners must switch to fuels with no more than 0.5 percent sulfur content, down from the current 3.5 percent, or install exhaust cleaning systems that achieve the same reduction, Shell noted in a brochure for customers.

There are very good reasons to cut sulfur: it contributes to both ozone depletion and acid rain, and it can cause or exacerbate respiratory problems.

But as a 2009 paper in Environmental Science & Technology noted, limiting sulfur emissions is a double-edged sword. “Given these reductions, shipping will, relative to present-day impacts, impart a ‘double warming’ effect: one from [carbon dioxide], and one from the reduction of [sulfur dioxide],” wrote the authors. “Therefore, after some decades the net climate effect of shipping will shift from cooling to warming.”

Sulfur pollution from coal burning has a similar effect. Some studies suggest that China’s surge in coal consumption over the last decade partly offset the recent global warming trend (though coal does have a strong net warming effect).

It’s difficult to estimate how much the new rule could affect temperatures. We don’t know enough about cloud physics and the behavior of atmospheric particles, nor how diligently the shipping industry will comply with the new rule, says Robert Wood, a professor of atmospheric sciences at the University of Washington.

Another wrinkle is that ships emit other particles that can sometimes also stimulate cloud droplets to form, including black carbon, a major component of soot. Removing the sulfur from the fuel could alter the size and quantity of these particles, which could affect clouds as well, says Lynn Russell, a professor of atmospheric science at the Scripps Institution of Oceanography.

“So we can’t really say exactly what the change will be,” says Russell, though she adds that the rule change is “likely” to produce a warming effect on balance.

The Hockey Schtick points to a new paper that finds a large increase in sunshine over the last 26 years.

“According to the authors, “the average increase of [surface solar radiation] from 1982 to 2008 is estimated to be 0.87 W m−2 per decade,” which equates to 2.26 W m-2 over the 26 year period. By way of comparison, this forcing was 12.5 times greater than the surface forcing alleged by the IPCC from increased CO2 over the same period.

“since the late 1980’s Rs has brightened over Europe due to decreases in aerosols but dimmed over China due to their increases. We found that variation of cloud cover determines Rs at a monthly scale but that aerosols determine the variability of Rs at a decadal time scale, in particular, over Europe and China.”

The warming since 1980 was caused by clean air legislation in the the US, UK and Europe.